Short post of the month February 2016 – gravitational waves

The subject of February’s short post of the month is gravitational waves. These were predicted by Albert Einstein back in 1916, and after decades of searching have finally been detected. In an announcement made on 11 February at a Washington DC press conference David Reitze, the executive director of the LIGO Laboratory, said:

Ladies and gentlemen, we have detected gravitational waves. We did it!”

(Castelvecchi and Witze 2016). Later that day president Obama tweeted his congratulations to the team:

Obama LIGO Tweet

What are gravitational waves?

Gravitational waves are ripples in space time. As gravitational waves pass through an object they cause it to move slightly. The Universe is believed to be awash with gravitational waves, because when massive objects move, such as the Earth orbiting around the Sun, they emit gravitational radiation. However compared to other forms of radiation such as light and radio waves, gravitational waves are very very weak, which is why they have proved so difficult to detect.

Gravitational waves

Image from NASA

The gravitational waves detected were due to the one of most violent events in the Universe, the merger of two black holes, which were rapidly rotating around each other.  Even so, the signal was still incredibly weak

How were they detected?

In the LIGO facility a laser beam is split into two and travels down two 4km tunnels which are at right angles to each other. The two beams then reflect back and forth many times  between two mirrors before they are eventually recombined at an electronic light detector.

Ligo

The apparatus is very finely tuned so that the waves from the two light beams, shown as  A and B in the diagram below, are completely out of phase with each other and as a result cancel each other out completely when they recombine giving no net signal at the light detector C.  Some of you may remember from your high school science lessons this is known as “destructive interference”.

Destructive Interference

When gravitational waves pass through the LIGO facility, the waves cause the tunnels to change their shape by a minute amount.  As a result of this, the distance travelled by each beam of light also changes very slightly, so that they are not completely out of phase and when the beams recombine they no longer completely cancel out. This produces a small signal at the detector.

On 14 September the same signal was found at the two separate LIGO detectors: Livingston in Louisiana first and Hanford in Washington State 7 milliseconds later. The fact that the patterns of the signals were the same and that there was a time delay between the two detections provided the proof that the signals were due to gravitational waves.

LIGO USA

What does this mean for astronomy?

Up until now astronomers have only been able to see the Universe by detecting electromagnetic radiation through telescopes which work at different wavelengths, for example those of visible light, radio waves and x-rays. The instruments at LIGO allow astronomers to observe the Universe in a whole new dimension, in effect to “feel” the Universe vibrating.

That view was reinforced by Stephen Hawking, who in an interview for BBC News (2016) said

“Gravitational waves provide a completely new way at looking at the Universe. The ability to detect them has the potential to revolutionise astronomy. This discovery is the first detection of a black hole binary system and the first observation of black holes merging.”

References

BBC (2016) Einstein’s gravitational waves ‘seen’ from black holes, Available at:http://www.bbc.co.uk/news/science-environment-35524440 (Accessed: 23rd February 2016).

Castelvecchi, D. and Witze, A. (2016) Einstein’s gravitational waves found at last,Available at: http://www.nature.com/news/einstein-s-gravitational-waves-found-at-last-1.19361 (Accessed: 21st February 2016).

 

 

10 thoughts on “Short post of the month February 2016 – gravitational waves”

  1. I was so bowled over by this news, thinking this wouldn’t happen in my lifetime. I’m so happy that it did. I understand another ‘LIGO’ is being built in Italy.

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    1. An interesting thought…

      In general whenever an object accelerates it will emit gravitation radiation. However, the answer to your question is that gravitational waves are so weak compared to other forms of radiation that only the most powerful celestial events generate gravitation waves which are detectable

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  2. A lot of news coverage was dedicated to this discovery without explaining why they were useful. So why are they useful? Also I assume they’re not like radio waves, constantly being emitted. So do we only get one chance to notice them?

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    1. Thanks for your comment Worzel,
      You raise an interesting point. The answer to your question is that, according to Einstein’ theory of general relativity gravitational waves are emitted whenever an object moves in a gravitational field.
      However, the strength of gravitational radiation caused by the Earth moving around the Sun, for example, is so weak that it would be impossible to detect. It is only the most violent celestial events that will emit gravitational waves strong enough for us to be able to observe.
      Gravitational waves were predicted by Einstein in 1916, but Einstein himself thought they would far too weak to be detected. Their discovery has provided additional confirmation that his theory of general relativity is correct.

      The Science Geek

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  3. Thanks for the info, TSG. I just can’t wait what comes next from this.

    “As gravitational waves pass through an object they cause it to move slightly..” I wonder if someday, somehow, we can amplify it. Like music. 🙂

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